Criminal Law

What’s the Difference Between Per Se and Impairment DUIs?

By John McCurley, Attorney
Read about the two types of standard DUI charges—per se and impairment.

In every state, there are two types of DUI (driving under the influence): “per se” DUIs and DUIs based on impairment. The difference between the two is how the prosecution proves you were “under the influence.” You can be convicted of a per se DUI if the prosecution shows you drove with an amount of alcohol or drugs in your body that exceeded the legal limit. Impairment DUIs, on the other hand, require proof that the alcohol or drugs that you ingested actually affected your mental or physical abilities.

This article discusses the differences between per se and impairment DUIs.

(To learn about the other elements that the prosecution must prove in a DUI case, see What Does the Prosecution Have to Prove in a DUI Case?)

Per Se DUIs

The laws of all states outlaw driving or operating a vehicle with a blood alcohol concentration (BAC) of .08% or more. Some states, including Illinois and Georgia, also have “zero tolerance” laws that make it illegal to drive with any amount of an illegal drug or controlled substance in your body. (625 Ill. Comp. Stat. Ann. 5/11-501 (2016); Ga. Code Ann. § 40-6-391 (2015).) If evidence shows that you were driving with more than the legal limit of alcohol or drugs your blood, prosecutors can charge you with a per se DUI.

To prove a per se DUI, prosecutors don’t need to show that you were drunk or your ability to drive was actually impaired. All the prosecution has to prove is that you were driving with a BAC of .08% or more; or in states with zero tolerance drug laws, that you had any amount of drugs in your blood.

Example: Mimi kicked off her Mardi Gras celebration with several early morning mimosas. She figured she was okay to drive because she started every day with several mimosas. But as she turned onto Bourbon Street, Lieutenant Colson noticed Mimi’s left turn signal wasn’t working. He pulled Mimi over, and as he approached Mimi’s window, he smelled alcohol. Mimi told Colson she’d had a few drinks, but that she certainly wasn’t drunk. Colson agreed that Mimi didn’t appear to be impaired—her speech wasn’t slurred and she passed several field sobriety tests with no trouble—but because of the alcohol odor, he gave her a breathalyzer test anyway. Mimi had a BAC of .10%. Although she might not have been impaired, Mimi is likely guilty of a per se DUI because her BAC was over .08%.

(For more information on per se DUIs, see What Is DUI Per Se?)

Implied Consent Laws

All states require drivers arrested for DUI to submit to chemical testing (most often the driver’s choice of a breath or blood test), for the purpose of determining the amount of alcohol (or drugs) in the driver’s body.

These laws are known as “implied consent laws,” on the theory that upon asking for permission to drive (that is, asking for a driver’s license), the applicant has impliedly consented to such a test when asked.

Drivers who refuse chemical testing typically face administrative consequences that might include license suspension, paying a fine, or installation of an ignition interlock device (IID). These consequences are usually in addition to those imposed for a DUI conviction. And in a handful of states, refusal to take a breath test can be charged as a separate crime. So drivers who refuse breath tests in these states can be convicted of two crimes: DUI and refusal to submit to chemical testing.

(You can read more about the consequences of refusing a chemical test in our article, What Happens If I Refuse to Take a Chemical Test After Being Pulled Over for DUI?)

Impairment DUIs

Unlike a prosecution based on the presence of a minimum amount of drugs or alcohol in the driver’s body, an impairment-based prosecution focuses on the quality of the driving and the driver’s behavior. Unlike the situation with a per se case, the term “impairment” has no clear-cut definition. For this reason, these cases are often harder for prosecutors to prove.

Now, what does the term “impairment” mean? The prosecution will need to present evidence showing that your abilities were negatively affected by the alcohol and/or drugs that you’ve ingested. Evidence of impairment might include erratic driving, poor performance on field sobriety tests, slurred speech, and odd behavior.

Second, how greatly impaired must the driver be to sustain a conviction? States differ on this issue. For instance, in California and Connecticut, you can be convicted of a DUI if you drive while impaired to “an appreciable degree.” (People v. Canty, 32 Cal. 4th 1266 (2004); Infeld v. Sullivan, 151 Conn. 506 (1964).) But in a number of other states, you can be convicted of a DUI even if you were impaired only to the slightest degree. In Florida, for example, the prosecution needs to prove you were impaired only to “some degree” to get a DUI conviction. (DiPietro v. State, 992 So. 2d 880 (2008).)

(For more information on impairment DUIs, see What Is an "Impairment" DUI?)

Talk to an Attorney

The DUI laws of all states are different, and the facts of each case unique. If you’ve been stopped for a DUI, get in contact with a local DUI attorney who can talk to you about what you’re facing and how best to handle your case.

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